The blow-by gas that leaks into a crankcase chamber via a gap between a piston and a cylinder of an internal combustion engine such as an automotive engine contains a large amount of hydrocarbons (HC). Hydrocarbons are a known cause of photochemical smog, and it has been widely practiced to return the blow-by gas to the intake system to burn the hydrocarbons with the mixture (blow-by gas recirculation system), instead of releasing the hydrocarbons to the atmosphere. The blow-by gas contains minute particles of engine oil, and it is desired to separate the engine oil to return the engine oil back to the engine. Various different types of oil separators are known such as inertia collision type, labyrinth type and cyclone type.
An inertia collision type oil separator is internally provided with a collision plate that blocks the flow of blow-by gas. When blow-by gas collides with the collision plate, the oil particles contained in the blow-by gas adhere to the collision plate owing to the inertia of the oil particles, and can be collected. Patent Document 1 discloses an inertia collision type oil separator, for instance. In this prior oil separator, a sheet spring that closes an opening of a flow passage is supported in the manner of a cantilever. As the sheet spring resiliently deflects, a gap is created between the opening and the sheet spring. The blow-by gas is accelerated as the blow-by gas flows through this gap, and collides with a wall placed in the downstream part of the flow passage. As a result, the oil particles contained in the blow-by gas adhere to the wall owing to the inertia of the oil particles, and are trapped on the wall. In this oil separator, as the blow-by gas collides with the wall at high speed, even oil particles of relatively small sizes can be trapped.
In a labyrinth type oil separator, a plurality of partition walls are positioned in a flow passage of blow-by gas in the manner of a maze. As a result, the oil particles are forced to travel long distances so that some of the oil particles are caused to drop to the bottom of the flow passage under the gravitational force. Also, as the flow direction of the blow-by gas is changed by the partition walls, the oil particles collide with the partition walls owing to the inertia of the oil particles, and are thereby trapped by the partition walls. Thus, it can be said that a labyrinth type oil separator is incorporated with a mechanism of an inertia collision type oil separator. Patent Document 2 discloses an oil separator of this type. In this prior oil separator, a plurality of projections project from the walls forming a labyrinth passage so that the oil particles in the blow-by gas are caused to collide with these projections, and trapped by these projections. By increasing the area of the part which the oil particles collide with, the efficiency of oil separation can be improved.